I. Field of the Invention
This invention relates generally to apparatus for long-term monitoring of the physiologic condition of a patient, and more particularly to an implantable or external device incorporating a microprocessor, a memory and an accelerometer for detecting motion due to patient activity as well as motion components relating to respiratory and cardiac rhythms, and from which critical hemodynamic diagnostic information can be derived.
II. Discussion of the Prior Art
In long term treatment of patients having cardiac abnormalities, it is important to monitor the cardiac performance over prolonged periods to assess the efficacy of any pacing or drug therapy being rendered to that patient. While so-called Holter monitors can be used to record ECG waveforms for later playback and evaluation, the amount of information that can be obtained from the ECG waveforms is necessarily limited.
It is known that data relating to the heart's mechanical functioning as a pump can be derived from heart sounds. Variability in heart sounds can provide insight into a patient's hemodynamic status. Arterial pulse pressure and various other parameters such as pre-ejection period that can be used by a physician in programming a dual chamber pacemaker for optimizing its AV delay parameter for a given patient may also be obtained from heart sounds. The timing to second heart sound may also be used to govern the rate response called for by an activity based rate responsive pacemaker. Also, heart sounds can be used to detect the occurrence of systolic and diastolic murmurs associated with valvular insufficiency or regurgitation. For example, a loud late mitral component of heart sound S.sub.1 is the hallmark of hemodynamically significant mitral stenosis. When S.sub.1 is loud, it is always associated with a loud opening snap, and the intensity of the snap correlates best with valve mobility. When calcification of the mitral valve occurs, the valve is stenosed and hence S.sub.1 is soft, and the opening snap is absent. In addition, high frequency heart sounds detected during diastole may contain information on occluded coronary arteries. See "Accelerometer Type Cardiac Transducer for Detection of Low-Level Heart Sounds" by Padmanabhan et al., IEE Trans Biomed Eng., 1993 Jan.; 40(1):21-28. It is also known that monitoring respiratory function in heart failure patients can identify patients with abnormal breathing patterns such as Cheyne-Stokes. The frequency of sleep apneas can be documented as well.
The use of movement registration for daily physical activity assessment is important to determine patient status. Standard laboratory-based exercise tests often used to define prognosis in patients with chronic heart failure do not relate to measures of normal daily activity. See Walsh JT et al., "Relation of Daily Activity Levels in Patients With Chronic Heart Failure to Long-Term Prognosis", Am J. Cardiol, 1997 May 15;79(10):1364-1369.
Thus, a need exists for a monitor that not only gathers and stores data over extended periods relating to the heart's electrical performance, but also its mechanical performance. In addition, there is also a need to monitor a patient's respiratory function and activity profile. It is a principal object of this invention to meet these needs.